ZoologicalJoumal of the Linnean Society, 64: 87-102. With 9 figures
October 1978
Thermoregulation in orb-web spiders :
new descriptions of thermoregulatory postures
and experiments on the effects of posture and
coloration
MICHAEL H. ROBINSON AND BARBARA C. ROBINSON
Smithsonian Tropical Research Institute, P.O. Box 2072,
Balboa, Canal Zone, Panama
Acceptedfor publication October 1976
Ideally, diurnal orb-web spiders should be able to ignore problems of insolation when siting their
webs and should be able to operate such webs without the necessity of retiring into shade. Postures
that minimize the surface area of the spider exposed to insolation may help to free the spider from
the danger of overheating. Such postures are here described for the first time for Argiope argentata
and three species of Gaderacantha. Experiments with dead Nephila clavipes show that postures
described as thermoregulatory do, in fact, reduce absolute temperatures (and the rate of
temperature increase) compared to normal predatory postures.
Metalic or other reflectant coloration occurs in many species of diurnal orb-web spiders. These
are partly listed herein. Experiments with one such species, Argiope argentata, show that overpainting
the silver parts increases both the rate of temperature increase and the absolute temperatures
reached when naturally coloured and black-painted spiders are exposed to the same radiant heat
sources.
I t is suggested that these results on the probable thermoregulatory function of metallic coloration
can provide insights into the probable habitat distributions of species whose coloration is known but
whose ecology is presently unknown. The general question of adaptive coloration in spiders is
discussed in the light of these results.
CONTENTS
. . . . . . . . . . .
Introduction
Materials and methods
. . . . . . .
Studies of postures
. . . . . .
Experimental studies
. . . . . .
Results
. . . . . . . . . . . . .
Thermoregulatory postures
. . . .
Web orientation
. . . . . . .
Experiments on internal temperatures
.
Discussion
. . . . . . . . . . . .
Acknowledgements
. . . . . . . . .
References
. . . . . . . . . . . .
. . .
. . .
. . .
. . .
. . .
. . .
. . .
.
.
.
.
.
.
.
.
. . .
. . . .
. . . .
. . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. . .
. . . .
. . . .
. . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. .
. . .
. . .
.
.
.
.
.
.
.
.
. .
.
. .
. .
. .
.
.
.
87
89
91
91
94
94
97
97
99
102
102
INTRODUmION
Web-building spiders that operate their webs from an exposed position at the
hub of the web are particularly vulnerable to the dangers of insolation and
0024-4082/78/10008 7 + 16/$02.00/0
7
R7
0 1978 The Linnean Society of London
88
MICHAEL H. ROBINSON AND BARBARA C. ROBINSON
consequent overheating. Such spiders can escape from insolation by moving off
the web into shade but are then forced into a suboptimal web-monitoring
location. The spider could also avoid insolation by building its web in
permanently shaded locations but this process might deny it access to potentially
fruitful prei-capture sites. Ideally the spider should be able to mitigate the effects
of insolation while retaining the ability to operate the web from the hub and not
losing the capacity to exploit unshaded web sites. Araneid spiders probably
achieve this end by making postural adjustments relative to the sun's position. In
addition some species may utilize reflectant coloration to reduce heat input.
Several authors have shown that web-building spiders adopt positions that
probably serve to minimize insolation when they are exposed to bright sunlight
(Pointing, 1965; Krakaeur, 1972; Robinson & Robinson, 1973, 1974). We here
describe therrnoregulatory postures found in Argzope argentata (Fabricius) and in
several species of Gateracantha. These new instances of postural thermoregulation are of interest since the Gateracantha species are dorsoventrally
flattened and laterally elongate, being rather untypical in shape for an araneid
spider. Their thermoregulatory postures are similarly bizarre but result in the
exposure of a minimum cross-sectional area to the sun. O u r new data for Argiope
argentata are of interest because the species has the greater proportion of its
dorsal surface a metallic silver colour. This surface certainly reflects radiant heat
(see below).
Postural thermoregulation is not confined to spiders and is found in insects
where postural changes can reduce the surface area exposed to insolation by as
much as five-sixths (see review in Wigglesworth, 1965). Robinson & Robinson
(1974) claimed a potential reduction in insolation of over four-fifths in the case
of Nephila clavipes (L.). Despite this there has been n o direct proof that postures
claimed to be thermoregulatory (in spiders) actually d o reduce the heat load. We
here report o n experiments where dead Nephila clavipes were oriented to radiant
heat sources so as to (a) maximize and ( b ) minimize, the surface area exposed to
heating while the internal temperature was monitored. These experiments
showed that simulated thermoregulatory postures reduced both the rate of
internal temperature increase and absolute level of temperature increase.
Arachnologists have suggested that araneid (and other) spiders may gain
protection from overheating caused by insolation by having silver coloration
(Robinson & Robinson, 1974; Levi, 1975). A surprising number of web-building
spiders are silvery o r light coloured o n their dorsal surfaces (Table 1 lists those
that we have seen o r for which good colour pictures have been published). To
test whether silver coloration affects radiant heat absorption we carried out
experiments with dead Argzope argentata (Fig. 1). We simply monitored the
internal temperature of spiders exposed to radiant heat and then painted the
silver parts black and repeated the observations. We were thus able to
demonstrate a considerable effect of coloration on the internal temperature of
the spider.
Data o n naturally occurring Argiope argentata webs show that their orientation
is apparently not restricted by thermoregulatory considerations. A similar
conclusion was arrived at by Robinson & Robinson (1974) after a study of the
web sites of Nephila clavipes. Argtope argentata may be freed of such constraints on
web siting by the possession of a repertoire of thermoregulatory postures and by,
additionally, having the benefit o f a heat-reflecting dorsal surface.
THERMOREGULATION IN ORB-WEB SPIDERS
89
The results of our experiments on the effect of coloration on heat absorption
allow us to make predictions about the geographical and habitat distribution of
some araneid species whose coloration is well known, but for which present
distribution data are inadequate. We also use our results as a starting point for
reviewing the broad question of the adaptive function of colour patterns in
araneid spiders.
Table 1. Colour distribution on araneid spider (see text)
~~
Colour pattern
Species
Source of information
Dorsal surface of cephalothorax
silver
Substantial unbroken area (> 1/2)
of dorsal abdomen silver
Abdomen mostly dark below
Cyclosa insulana (Costa)
(immatures only)
Argiope argentatan (Fabricius)
Argiope savignyi" Levi
ArgiopeJorida* Chamberlain & h i e
Argiope lobata (Pallas)
Argiope blanda 0. P. Cambridge
Argiope extensa Rainbow
Argiopeprotasa L. Koch
Argiope symatica L. Koch
P a s . obs.
Pers. obs.
Pers. obs.
Pers. obs.
Levi(1968)
Mascord (1970)
Mascord (1970)
Mascord (1970)
Dorsal surface of cephalothorax
silver
Dorsal abdomen with substantial
area of white bands
Abdomen mostly dark below
Argiope aemulan (Walckenaer)
Argiope aetheria* ( Walckenaer)
Argiope reinwardti * (Doleschall)
Argiopepicta* L. Koch
Argiope tniaciata Forskoel
Argiope bruennichi (Scopoli)
Argiope amoena L. Koch
Argiope minuta Karsch
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Yaginuma (1968)
Yaginuma (1968)
Dorsal surface of cephalothorax
silver
Abdomen pale coloured above
Abdomen mostly dark below
Argtope ocyalotdes L. Koch
Herennia omatisstma (Doleschall)
Nephtla omata Rainbow
Nephtla edults (Labillardiere)
Pers. obs.
Pers. obs.
Mascord (1970)
Mascord (1970)
Thorax not silver
Dorsal surface of abdomen pale and
glossy for the most part
Mainly dark below
Carteracantha cancrqomis (L.)
Gateracantha arcuata (Fabricius)
Gateracantha curvispina GuPrin
Gateracantha taeniata (Walckenaer)
Gasteracantha theisi GuCrin
Micrathena sagattata (Walckenaer)
Micrathena sexspinosa (Hahn)
Isoxya spp.
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Pers. obs.
Abdomen with large areas of silver
o r white, both dorsally
and ventrally
Leucauge papuana" Kulzynski
Cyrtophora moluccemis * (Doleschall)
Cyrtophora cicatrosa (Stoliaka)
Pers. obs.
Pers. obs.
Y. D. Lubin, pers. obs.
Pers. obs.
~~
MATERIALS AND METHODS
The research described herein falls into two main parts. The observational and
manipulative studies on postural thermoregulation in Argiope argentata and the
experimental studies in which internal temperatures of dead spiders were
measured. As a descriptive convenience we separate these two parts below.
90
MICHAEL H. ROBINSON AND BARBARA C. ROBINSON
Figure I . Adult female Argtope argentala seen from the dorsal surface (by permission Smithsonian
Institution Press).
THERMOREGULATION I N ORB-WEB SPIDERS
91
Studies $postures
Determinations of thermoregulatory postures in Argzopes argentata were carried
out in the laboratory using a 100-Wreflector bulb as a heat source. Confirmatory
observations on free-living spiders were carried out in the field using redirected
sunlight (as described in Robinson 8c Robinson, 1974: 389). (Field work on A.
argentata is most readily carried out, in Panama, on the large wet season
populations but then sunny spells are intermittent and largely unpredictable.)
The observations on Gasteracantha theisi Guerin, G. taeniata (Walckenaer),and G.
breuispina Doleschall were carried out at the Wau Ecology Institute, Wau, Morobe
District, Papua New Guinea, during the period November 1973-September
1974. Natural history notes on the first two species are given in Robinson, Lubin
& Robinson (1974). All the work on Gasteracantha species was carried out in the
field using methods described in Robinson 8c Robinson (1974).The observations
on Argiope argentata and the Gasteracantha species were confined to adult female
spiders; the adult males are very small and do not spend long periods exposed to
dangers from insolation.
We also carried out a census of web-orientations adopted by Argiope argentata.
This was done by recording, in July 1976, the compass orientations of all A.
argentata webs found along both sides of an east-west stretch of deserted road on
the south bank of the Panama Canal close to Summit, Canal Zone. We have
plotted the results in the same way as those given in Robinson & Robinson ( 1974,
Fig. 4).
Experimental studies
All experiments involving the measurement of internal temperatures were
carried out at the Barro Colorado Island Research Station of the Smithsonian
Tropical Research Institute, during January-February 1976 (dry season). The
adult female spiders used in the experiments were collected in the Panama Canal
Zone and killed (by freezing) immediately prior to the experiments. To avoid any
effects of decomposition on internal temperatures we used only freshly killed
spiders. [We did not use live spiders because (a) there is no easy way to prevent
live specimens from bleeding at the point of insertion of the thermocouple, and
(b)we wanted to arrange the legs of the spider in more-or-less natural attitudes
and this would have been impossible with living specimens unless they were
anaesthetized-which would have further complicated procedure.]
Controlling the orientation ofthe spider
To passively orient the spider at different angles to the heat source we built the
perspex (U.S.=Plexiglas) device shown in Fig. 2. As can be seen from the
diagram the device is almost infinitely adjustable. The spider was mounted,
dorsal surface uppermost, on the perspex carrier with cyanoacrylate glue (Super
Glue 3, Dupont). The glue dries very rapidly and bonds insect and arachnid
cuticle most effectively to a variety of plastics and metals. Once the spider was
mounted on the carrier we could adjust its orientation to the sun o r an electric
radiant heat source. To adjust the orientation to the sun we used the cast shadow
of the apparatus and the spider as a means of standardizing the experimental and
control presentations. The postures designated “experimental” were those in
which the minimum cross-sectional area of the spider was exposed to insolation
MICHAEL H . R O B I N S O N A N D BARBARA C. R O B I N S O N
F
Figure 2. Apparatus used in experiments. The perspex disc ( D ) is hinged onto the metal support
z t m d IS). I t can be inclined at any angle. The finger (F) slides around the disc and carries the pin (p)
o n which t i i f spider is indirectly mounted. The thermistor ( t ) is shown inserted in the abdomen of a
\[)lilfT
(postures 1 and 2, of Robinson & Robinson, 1974: fig. 1). In the “experimental”
posture the disc of the apparatus lies parallel to the sun’s rays and thus we simply
adjusted the apparatus until the shadow cast by the disc was a single straight line.
The control orientation was that of maximum insolation, i.e., with the long axis
of spider at right angles to the sun. To achieve this it was only necessary to obtain
a perfectly circular shadow cast by the disc.
When using an incandescent heat source (100-W reflector bulb in porcelain
socket) we simply marked a point on the bench surface that was directly below
the centre point of the bulb, and placed the apparatus with the stand directly on
this point. With the apparatus placed thus the spider was directly below the
centre point of the bulb whether in the experimental or control position. We
adjusted the height of the apparatus so that the distance between spider and lamp
were the same irrespective of the orientation of the spider.
In the experiments on the effect of coloration on temperature we always used
the spider in the position of maximum heat input (at right angles to the sun or
incandescent heat source).
Changzng the dorsal coloration of the spider
We carried out these experiments by first testing the unaltered spider (the
control) and then painting the silver parts black. We used Pactra Matt Black
(model aircraft enamel paint) and allowed it to become touch dry ( 10 minutes)
before testing. Since there was a possibility that a layer of paint might produce a
T H E R M O R E G U L A T I O N I N ORB-WEB SPIDERS
93
change in the effect of radiant heat we painted some spiders silver on top of
existing silver parts and tested these (we could detect no effect of the paint per se).
Measuring the internal temperature
We measured the temperature within the abdominal cavity by inserting the
thermocouple from the apex of abdomen so that its tip was approximately
midway along the length and somewhere close to the centre. (Ifwhen the thermocouple was withdrawn it had been badly bent we would have assumed that
it did not lie in the right relationship to the body surface. This never happened;
the wire must have been stiff enough and the viscera soft enough to allow easy
penetration.) In the case of Nephila clauipes it was sometimes necessary to
penetrate the cuticle with a needle prior to inserting the thermocouple. Some
small loss of body fluid occurred at the point of insertion but quickly stopped.
After bleeding ceased we cleaned the surrounding area with wet tissue paper to
ensure that the spider was not discoloured. Bleeding never resulted in a
discernable alteration in the shape of the apex of the abdomen. (This collapses
conspicuously if excess bleeding occurs.) Some kind of coagulation must have
occurred around the point of entry of the thermocouple since fluid did not
subsequently leak out during the heating process when expansion must have
taken place.
We used thermocouples made by soldering together copper and constantan
wire (36 SWG) with the insulation removed and soldered portion less than 2 mm
long. These were used in conjunction with a S-B Systems (Manhatten, Kansas)
reading device (calibrated potentiometer). We used two thermocouples on the
same instrument, one placed in the spider and one reading air temperature in
the shade). We took temperature readings at half-minute intervals.
Experimentalprocedures
Experiments in the sun were carried out during the dry season on cloudless
days, using the period from around one hour before to one hour after the sun’s
zenith time. We simply set up a table in the laboratory clearing at Barro
Colorado Island and surrounded this by a 2-m high screen to eliminate
troublesome temperature fluctuations due to variable dry season winds.
Experiments using a heat lamp were carried out in a large air-conditioned room
with a heat source mounted 75cm above the table top. In each case, air
temperature was monitored 20 cm away from the spider, approximately at the
same height but with the thermocouple shaded from the radiant heat. Prior to
each experiment the spider’s internal temperature was allowed to equilibrate
with air temperature.
The posture experiments were carried out very simply. Each spider was tested
twice, once in the experimental posture, once in the control posture. Five spiders
were tested in sunlight and five under the incandescent heat source.
The colour experiments were carried out in a fixed order. The unpainted
condition. was always tested first. Again five spiders were tested in the sun and five
under the heat lamp.
All the experiments were open-ended in so far as they were continued until
there was no increase in the temperature differential (spider internal temperature
minus air temperature) for three successive readings.
94
MICHAEL H . ROBINSON AND BARBARA C. ROBINSON
RESULTS
Thermoregulatorypostures
Argzope argentata and Gasteracantha spp.
Functional equivalents of the postures described for Nephila clavipes and N .
maculata (Robinson & Robinson, 1973: 64-66; 1975: 20) occur in Argiope
argentata, Thus the spider is able to use postural thermoregulation irrespective of
the compass orientation of the web and its relationship to the directions of actual
insolation. The spider responds to dorsal illumination by adopting the posture
shown in Fig. 3. This is essentially posture 1 of Robinson & Robinson (1974:
fig. 1 ) . It differs from the Nephila clawipes posture only in that the thorax is not
really oriented parallel to the abdomen, there is a distinct angle between the two
parts of the body. When the sun shines through the web onto the dark ventral
surface of the body the spider quickly adopts the posture shown in Fig. 4. This is
an analog of posture 2 of Nephila clawipes (Robinson 8c Robinson, 1974: fig. 1 ) .
The legs 1 may be raised off the web when A . argentata assumes this posture, the
apex of the abdomen is pressed tight against the hub silk but the spider never
assumes a posture quite so extreme as the Nephila species (compare Fig. 4 with
fig. 30 of Robinson & Robinson, 1973). Posture 1 and 2 may be combined with
responses to lateral illumination to produce compound orientations.
In the field examples of posture 2 are frequently encountered and we have seen
them in several other species of Argiope (asterisked in Table 1). Since there is a
striking difference in the coloration of dorsal and ventral surfaces we measured
the time taken to assume the thermoregulatory posture when the spider was
heated dorsally and ventrally. The difference was striking; spiders heated
ventrally with a 100-W incandescent reflector lamp at 3 inches from the body
surface ’took from 2-3 minutes to move into a full thermoregulatory posture.
The same spiders heated with the same lamp at the same distance from the body
dorsally took from 10- 13 minutes to move into thermoregulatory posture.
As noted earlier most Gasteracantha species have opisthosomas that deviate
markedly from the more-or-less longitudinally elongate form of most araneid
spiders. Furthermore, unlike the species whose thermoregulatory postures have
been investigated, the web has an open hub. The species that we studied all build
webs that are more distinctly aerial than those of Nephila clawipes, N . maculata, and
Argzope argentata. The open hub may function principally to allow the spider to
manipulate the tension of the web and particularly to suddenly release the web
tension after a prey item strikes it, thus helping to enmesh the insect. In addition,
however, the open hub gives the spider the possibility of orienting its body
through the hub rather than merely beneath it. This facilitates postural thermoregulation. Gasteracantha species successfully orient the narrow edge of their
dorso-ventrally flattened body towards the sun and frequently direct the least
cross-sectional profile (the lateral one) directly at the sun. An extreme version of
the latter orientation is shown in Fig. 5. Support in this “clothesline” posture
may be partly obtained from the dragline, as shown in the figure. All the postures
that we observed involved the spider hanging below the web (which is often
closer to horizontal than vertical) although part of the opisthosoma may
protrude above the web through the open hub. Even when the lateral edge is not
exactly aligned with the sun, the spider has a good edgewise orientation and must
expose a much smaller surface area to insolation than would be the case if the
sun struck the dorsal (or ventral) surface at right angles.
THERMOREGULATION I N ORB-WEB SPIDERS
Figure 3. Adult female Argiope argatatu subject to radiant heating from above the dorsal surface. The
apex of the abdomen is pointing directly at the heat source (white arrow): note the distinct angle
between abdomen and cephalothorax.
95
96
MICHAEL H . ROBINSON AND BARBARA C. ROBINSON
Figwe 4 . Adult female Argiope argenfda subject to radiant heating, through the web onto the ventral
\urfitce (white arrow). The apex of the abdomen is pointed directly at the heat source.
THERMOREGULATION I N ORB-WEB SPIDERS
97
Figure 5 . Gusteracanthabreuispina orienting to minimize insolation. The web is more or less horizontal
and the sun (white arrow) is shining through the web from the left side of the picture so that the
spider is, in effect, pointing the right-hand edge of its abdomen at the sun. It is hanging supported
from the edge of the open hub by its right leg IV and holding its dragline (right of picture) with its
left legs.
Web orientation
Figure 6 shows the compass orientation of Argiope argentata webs in open grassy
verges on both sides of more than 100 m of eastlwest road. The side of the web
on which the spider was sitting is indicated on the diagrams and from this the
slope of the web relative to the forest edge (shade) can be deduced. (Since the
spider is always on underside of the sloping web.)
July 1976 was unusually dry and sunny and we would conclude that the spiders
are siting their webs wherever there are suitable web supports and that the
orientation is not conspicuously restricted by thermoregulatory factors.
Experiments on internal temperatures
The results are very striking indeed. Each animal is its own control and in each
case there was no overlap between experimental and control curves. We therefore
feel justified in plotting the means for each point (Figs 7 and 8). From Fig. 7 it can
be seen that with both sunlight and incandescent heating postural thermoregulation works to reduce the rate of temperature increase and the absolute
level attained within the spider. Similarly as compared with normal coloration
the black-painted spiders heat more rapidly and to a higher level.
MICHAEL 1-1. ROBINSON AND BARBARA C. ROBINSON
i
Figurc 6. Distribution of webs along two sides of an easthest road. Webs are shown distributed
about it central point but were, in fact, scattered along the verges. The two lines show the
I-rlatioristiipof the webs to shade (forest edge). The dots indicate the side of the web on which the
apidrr-restrd. Noteworthy is the fact that the orientations on each side of the road are approximately
I)alarrced (see text for details).
Time ( m i d
Figure 7 . Graph of postural thermoregulation results; each point is an average of the results from
five tests. The endpoint is the fint of three results for which there was no change. The temperature
plotted is the difference between the internal temperature of the spider and ambient.
THERMOREGULATION I N ORB-WEB SPIDERS
99
8-
7-
/o-oSun-black
,.-•
/.,.-•
DISCUSSION
It could be objected that since our internal temperature results were obtained
from dead spiders they may be of little relevance to the situation in living spiders,
where circulation of the blood could move heated fluid to regions of greater or
lesser thermal inertia and affect the situation in the abdomen. This is certainly
true although it is difficult to even guess whether circulation of the blood would
serve to increase the abdominal temperature or, conversely, to decrease it. It is
interesting to note that the absolute differences (which we would regard as far
less important than the relative differences) fall within the range of those
measured in live insects using postural thermoregulation (Wigglesworth, 1965:
602-603). Furthermore Uvarov ( 1948) reported an “in sun” temperature
difference between black and green hoppers of Locusta migratoriu of 6.6OC.
The difference in the time taken to assume thermoregulatory postures when
heated from the ventral and dorsal surfaces by an incandescent lamp is also
consistent with the view that the silver surface reflects heat. (This result, of course,
was obtained from living spiders.)
It is interesting to note that in Panama there are two species of Argiope: A.
argentata and A. sauignyi. The latter first described in 1968 (Levi, 1968) is little
known but most of the individuals that we have seen have been in shaded forest
clearings or along forest trails. Argiope sauignyi (Fig. 9) has a much greater area of
its dorsal abdomen silvery white than has the Silver Argiope, Argzope argentuta.
The proportions of silver to total area for the two species are: A . argentata
1 : 1.96 and A. sauignyi 1 : 1.30. On the basis of our experiments on the function
of silver coloration we would suggest that the proportionally greater silver area is
a thermoregulatory adaptation, and that A. sauignyi must occur in habitats that
receive considerable insolation. This is not consistent with our present
I00
MICHAEL H . ROBINSON AND BARBARA C. ROBINSON
Figure 9 Adult female Argrope S Q U Z p y viewed from the dorsal surface showing the extensive silver
dieas on the cephalothorax and abdomen. Compare with Fig. 1 .
knowledge of the species but the rare habitat data may result from spiders being
most readily found in suboptimal habitats. If it were a forest canopy species:
1. It would be subject to more insolation than A . argentata.
2. Spiderlings might finish up at the forest floor after dispersal, and some of
these could succeed, mature, and provide the rare records mentioned
above.
3. It would probably be subject to intensive predation from insectivorous
birds and mammals.
The species is extremely “nervous” and jumps out of its web (as an escape
behaviour) much more readily than A . argentata, whose defensive behaviour
rarely involves jumping from the web (Robinson 8c Robinson, 1970: 650).Such
“nervousness” is more readily explicable if the spider is a canopy species rather
than an understorey one. Adult and preadult (penultimate instar) A . sauignyi
THERMOREGULATION IN ORB-WEB SPIDERS
101
females build complex disc stabilimenta quite frequently. This phenomenon is
not seen in A . argentata, or any Argiope species that we know (Robinson &
Robinson, 1970; 1974). Most species build such discs only as juveniles (see also
Kaston, 1964). Discs may act as shields that reinforce the hub against probing
predators (hummingbirds, for instance) since the spiders can shuttle quickly
behind such discs from one side of the hub to the other (Robinson 8c Robinson,
1970: 650; see also Tolbert, 1975); discs could also function as sunshields. The
unique persistence of disc-building into late developmental stages may be further
circumstantial evidence for a canopy habitat for A . savignyi.
Figures on the amount of insolation that occurs at different latitudes are
extremely difficult to come by. This is largely due to the absence of good data for
the tropics. Conventional wisdom suggests that the greater part of the tropics are
cloudy for the greater part of each year whereas certain other regions,
particularly Mediterranean scrub forest areas, may be much less cloudy. Satellite
data on cloudiness (Sadler, 1969) show that even when cloudiness is averaged
over wide areas there is a striking difference between cloudiness at low and
middle latitudes (Sadler, 1969: table 4, 12-17). If Mediterranean habitats have
the most insolation we would predict that a larger proportion of diurnal araneids
from these regions should have large reflectant areas on their bodies, at least in
the absence of other thermoregulatory adaptations. The distribution of Argzope
lobata (Pallas)seems to fit this prediction.
The functional interpretation of the colours of diurnal orb-web spiders is a
most interesting and largely unexplored field. Robinson 8c Robinson (1970: 650)
in a discussion of defensive adaptations in araneids point out that camouflage in
the strict sense of background matching “is not so easy for animals which
inevitably sit above the background (in their webs)”. It is achieved by Iierrenia
ornatissima which builds webs very close to treetrunks and rockfaces (Robinson 8c
Robinson, 1973: fig. 25) and by many Cyclosa species which build “artificial”
backgrounds of detritus into their webs. But most araneids cannot rely on
camouflage per se and may have evolved colour patterns that are shape (outline)
concealing or obliterative/disruptive. Striping and spotting are classical cases of
obliterative/disruptive coloration (Cott, 1940; Edmunds, 1974) and are
widespread in spiders. Unlike background matching the requirements of
obliterative coloration do not necessarily conflict with the possession of areas of
light coloured or metallic reflectant pigmentation. Striping on a silver
background may be an effective outline concealment device, it certainly occurs in
many Argzope species. This may explain why the spider is not entirely silver in
coloration. Some species are almost entirely silver on their upper surfaces but
dark beneath (see Table 1). This may have something to do with web orientation
but this is by no means clear at the present state of knowledge. (It is noteworthy
and suggestive that some Leucauge species, that rest ventral surface uppermost in
more-or-less horizontal webs, have silver on their ventral survaces, Leucauge
papuana is an example.)
The possession of glossy light colours, principally white and yellow, by species
of Gusteracantha and Micrathena may be another instance of multi-functional
coloration. In some cases these areas of glossy “enamel-like’’ colour are not
broken up by stripes to form obliterative patterns. They may combine the
function of heat reflection with warning coloration. The latter is a possibility
since several species have strong sharp spines which may be antipredator devices.
Robinson ( 1969) suggested that the bizarre shape of Gusteracantha species could
102
MICHAEL H . ROBINSON AND BARBARA C. ROBINSON
act as an antipredator adaptation by capitalizing on the predator's negative
response to extreme novelty. Bright colour could reinforce the effect of shape in
this respect. We would again emphasize that the bright glossy coloration of these
unusual spiders is restricted to the dorsal surface of the abdomen. This
phenomenon deserves investigation.
Future studies of adaptative coloration in araneid spiders must consider heat
reflection as an important potential function particularly in species that assume
predatory positions at the hub of the web by day. Studies of coloration could
profitably include comparisons between nocturnal and diurnal species, between
species living under different climatic regimes and between species with different
habitat preferences.
ACKNOWLEDGEMENTS
Prof. George Batholomew, University of California at Los Angeles, helped
with the loan of thermocouples and equipment and gave pertinent advice.
Donald Windsor, Smithsonian Tropical Research Institute, produced the graphs
on a Hewlett Packard 9830A computor and computed the surface areas of silver
from photographs of spiders, using that cornputor's digitizer attachment. We are
extremely grateful for this help.
REFERENCES
CO7-T. H.B., 1940. Adaptwe Colorallon i n Animals. London: Methuen.
EDML'NDS, M.,
1974. Oefnce in rlnimds. London: Longman.
KASTOX, B . J . , 1964, Theevolution ofspider webs. AmericanZoologtst, 4 : 191-207.
KKAKhl'ER, T .. 1972. Thermal responses of the orb-weaving spider :Vephila clauipvs (Araneac: Argiopidae).
.American .Ifidland .Vaturaltst, 88: 245-250.
LEYI, H. W., 196X. The spider-genera Gea and Argiope in America (Araneae:Araneidae). Bulletin ofthe Museum of
(.'rirriparntivr zoo lo^ af Haruard College, 136: 3 19-352.
LF.VI, H . W., 3975. The orb-weaver genera Verrucosa, Acanthepeira, Wagneria, Acaresia, Wixia, Scoloderus and
:lIpaida north ofblexico. Bulletin ofthe .Museumofcomparative Zoologv at Harvard College, 147: 35 1-39 1.
MASCORD, R.. 1970. Australian Spiders in Coloitr. Sydney: Reed.
POISTING, P. J , , 1965. Some factors influencing the orientation of the spider Fronlinrlla roinmunis (Hentz.)in
its web IAraneae Linyphiidae). Canadian Entomologist, 97: 69-78.
ROBINSON, M. H., 1969. Defenses against visually hunting predators. In tr~olutionaryB i o l o ~ y I, l l . New York:
Appleton, Certtury, Crofts.
ROBINSON, M . t i . , LUBIN, Y. D. & ROBINSON, B., 1974. Phenolog), natural history a n d species diversity
otwcl,-building spiders o n three transects at Wau, New Guinea. Pm$c Zn~ecfs.16: I 17-163.
ROBINSON, M. H . & ROBINSON, B., 1970. The stabilimentum of [he orb-web spider Argiope argentata
' Araneae: AraneidaeJ : a n improbable defence against predators. Canadian Entomologist, 102: 64 1-655.
ROBINSON, M. €3. & ROBINSON, B., 1973. Ecology and behavior of.the giant wood spider Nephila rnaculata
1 Fdbriciusi in New Guinea. Srnithonian Contributiorrr to zoo log^, 149: 1-76.
R O H I N S O N , 91. H . & ROBINSON, B. C., 1974. Adaptive complexity: the thrrmoregulatory postures of the
golden-\& spider .Vephila clautpes at low latitudes. American Midland N a t u r a h f , 92: 386-396.
SADLER, J . C., 1969. .4oerage Cloudintss in the Troptcsfrom Satellite Observations. Honolulu: East-West Centre
Press, Univ. Hawaii.
TOLBERT, W.W., 1975. Predator avoidance behaviors a n d web defensive str-uctures in the orb-weavers Argiope
mirarrlio and .4rgiopr Irfarn'ata ;Araneae. Araneidae). PJyche, 82: 29-52.
1.\.AROV. B., 1947. Recent advances in Acridology : anatomy a n d physioloQ 01' Acridzdae. Transactions ofthe
Rowl f:ritorrmlugtcu/ Society o/ London, 99: 1-75.
WI(XLESWORTH, V. B., 1965. The Principles ojlnsect Physiology. London: Methum.
Y A G I N I ' M A , T., 1969. Sptderi ofJapan i n Colour. Osaka: Hoikusha.